[docs] Revert FetchContent section (#27311)

* [docs] Mark FetchContent as experimental

* [docs] Revert 'FetchContent' section

It is not a supported, designed, or tested method and has several significant shortcomings.

README.md

@@ -33,7 +33,6 @@ you can run `vcpkg help`, or `vcpkg help [command]` for command-specific help.
     - [Vcpkg with Visual Studio CMake Projects](#vcpkg-with-visual-studio-cmake-projects)
     - [Vcpkg with CLion](#vcpkg-with-clion)
     - [Vcpkg as a Submodule](#vcpkg-as-a-submodule)
-    - [Vcpkg via FetchContent](#vcpkg-via-FetchContent)
 - [Tab-Completion/Auto-Completion](#tab-completionauto-completion)
 - [Examples](#examples)
 - [Contributing](#contributing)
@@ -263,28 +262,6 @@ This will still allow people to not use vcpkg,
 by passing the `CMAKE_TOOLCHAIN_FILE` directly,
 but it will make the configure-build step slightly easier.
 
-### Vcpkg via FetchContent
-
-You can also grab vcpkg with CMake's built-in [FetchContent](https://cmake.org/cmake/help/v3.24/module/FetchContent.html) module.
-
-Don't worry about the bootstrap scripts, since `vcpkg.cmake` will run them for you!
-
-```
-cmake_minimum_required(VERSION 3.14)
-
-include(FetchContent)
-FetchContent_Declare(vcpkg
-    GIT_REPOSITORY https://github.com/microsoft/vcpkg/
-    GIT_TAG 2022.09.27
-)
-FetchContent_MakeAvailable(vcpkg)
-
-# NOTE: This must be defined before the first project call
-set(CMAKE_TOOLCHAIN_FILE "${vcpkg_SOURCE_DIR}/scripts/buildsystems/vcpkg.cmake" CACHE FILEPATH "")
-
-project(FOOBAR LANGUAGES "CXX")
-``` 
-
 [getting-started:using-a-package]: docs/examples/installing-and-using-packages.md
 [getting-started:integration]: docs/users/buildsystems/integration.md
 [getting-started:git]: https://git-scm.com/downloads